Steam generators in pressurized water nuclear reactors generally incorporate a bundle of U-shaped tubes the ends of which are fixed in a tube plate. This tube plate divides the steam generator into a zone receiving pressurized water which forms the fluid bringing its heat to the steam generator and a zone receiving feed water to be vaporized in the steam generator. The tube bundle is arranged in the part of the steam generator which receives the water to be vaporized, and the ends of each of the tubes pass through the plate over its entire thickness so as to be placed in communication with the zone of the steam generator which receives the pressurized water or primary fluid. This zone forms a water box made of two parts one of which receives the pressurized water and distributes it into the tubes of the bundle while the other collects the pressurized water which has circulated in the tubes, before it returns to the nuclear reactor vessel. The feed water is heated and vaporized in contact with the outer wall of the tubes of the bundle.
The tube plates of steam generators in pressurized water reactors are very thick and can reach or exceed 0.60 meter. The ends of each of the tubes of the bundle are fixed by crimping in the holes passing through the tube plate over its entire thickness. This operation, also called expansion rolling, consists in rolling the wall of the ends of the tubes introduced into the tube plate with the aid of a tool called an expanding roller incorporating rolling wheels which is moved within the tube in all its part situated within the tube plate. The ends of the tube are welded to the tube plate at their end which is flush with the face of this tube plate which comes into contact with the primary fluid. The other face of the tube plate is crossed by the tubes which enter the zone of the steam generator which receives the water to be vaporized.
The tubes of the bundle form a dividing wall between the primary radioactive fluid and the secondary fluid consisting of the feed water or its vapor. This vapor is led away towards the turbines associated with the nuclear reactor and situated outside the reactor building which forms the containment enclosure of the latter. It is thus very important that the tubes ensure a perfect separation between the primary fluid and the secondary fluid.
When the steam generator is brought into operation, this perfect separation of the fluids is ensured, the integrity of the tube walls and the quality of the welds having been checked. However, after some period of operation of the steam generator, this is no longer necessarily the case, since cracks or perforations may have appeared in some of the tubes, in particular under the effect of corrosion. Steam generators are, in fact, intended to be used for very long periods, of the order of forty years, and despite the corrosion resistance of the materials employed in their construction, an attack on the tubes, which are generally made of a nickel alloy, can take place in some zones.
In particular, it has been found that the part of the tubes which is situated in the vicinity of the tube plate face which comes into contact with the water to be vaporized is subjected to greater corrosion than the other parts of the tube. This, in fact, is the part of the tube which contains the transition zone between the part which is distorted during the expanding operation and the undistorted part of the tube. In an operating reactor the primary fluid is at a temperature of approximately 325.degree. C. and a pressure of 155 bars. This fluid consists of demineralized water containing variable quantities of boron in the form of boric acid which absorbs neutrons and permits control of reactor power, and lithium hydroxide to maintain the pH of the primary fluid at a value which permits the corrosion to be limited. However, in the transition zone, where the residual stress concentration is high, after expansion rolling, in particular in the internal surface layer of the tube, corrosion of this tube takes place in contact with the primary fluid at a high temperature and high pressure, this corrosion being even capable of resulting in perforation or cracking of the tube, and consequently in entry of the primary fluid into the secondary fluid.
Attempts have been made to improve the corrosion resistance of steam generator tubes, in the transition zone, by relieving the stresses in the tube by diametral expansion. Thus, tools have been designed which make it possible to carry out rapidly and automatically the stress-relieving of the outer wall of the tubes of a steam generator in their transition zone. Since the expansion rolling of the tubes is carried out over the entire part of the tube within the tube plate, the transition zone is situated in the vicinity of the tube plate face which comes into contact with the feed water to be vaporized. This stress-relieving operation, which must be carried out on the ends of each of the tubes in the steam generator, is relatively time-consuming, even when tools whose operating cycle is entirely automatic are employed. In fact, a steam generator of a pressurized water nuclear reactor contains a very large number of tubes, possibly five thousand.
Furthermore, after the operation of relieving stresses in the outer skin of the tube, the stress concentration remains relatively high in the inner skin of the tube. Sensitivity to corrosion therefore remains higher in this zone of the tube close to the tube plate face in contact with the water to be vaporized.
The feed water is demineralized water containing hydrazine and ammonia for its conditioning in order to reduce its corrosive power. However, this feed water, which is subjected to phase changes and which is recycled to the steam generator after being condensed, attacks some parts of the secondary circuit and carries corrosion products which tend to accumulate on the upper face of the tube plate, on the secondary side of the steam generator. These corrosion products are deposited in the form of sludges which contain essentially magnetite and can accumulate to a height of several centimeters on the upper face of the tube plate, during the operation of the steam generator.
The part of the tubes of the bundle which is in the vicinity of this face of the tube plate suffers increased corrosion on its outer surface owing to the accumulation of impurities in contact with the tube, and in particular in the gap which can be present between the tube and the end of the hole in the tube plate, owing to poor circulation of the secondary fluid and to the poor heat exchange of this fluid in this zone, and finally because of the creation of an electrochemical environment which is unfavorable for the corrosion resistance of the tube.
To overcome these disadvantages, devices have been suggested which permit the layer of impurities on the upper face of the tube plate to be eliminated more or less completely. Despite this, corrosion of the tube on its outer surface, in the vicinity of the upper face of the tube plate, can be high and can increase the seriousness of the corrosive effect of the primary fluid inside the tubes.
There is also known, from French Pat. No. 2,484,875, a process for leakproof fixing of a tube in a tube plate, in which use is made of a leaktight sleeve placed around the tube in its part entering the tube plate, before expansion rolling, which makes it possible, in particular, to eliminate the residual annular space between the tube and the outlet end of the hole in the tubular plate. However, such a process complicates the expanding operations, because it requires a sleeve to be fitted around each of the ends of the tube before they are fitted in the tube plate. Finally, this process provides no protection for the inner surface of the tube.